Borehole imaging involves lowering a specialized logging tool equipped with imaging devices into a borehole. At selected depths in the borehole, the logging tool is rotated within the borehole to generate an image of the formation surrounding the borehole. In wireline logging, the logging tool is lowered into the borehole on the end of an electrical cable or wireline, where the wireline doubles up as a means of transmitting the acquired image data to the surface. The length of the wireline in the borehole also provides a direct measure of the depth of the logging tool in the borehole. Wireline logging can provide high-quality, high-density data quickly and efficiently, but there are situations where wireline logging may be difficult or impossible to run, such as while drilling the borehole, when the borehole is highly deviated or horizontal, or when the borehole is unstable. An alternative to wireline logging is logging-while-drilling (LWD). In a LWD environment, the logging tool is integrated into a drill string deployed in the borehole. Image data collected by the logging tool is stored in memory and retrieved when drilling stops and the drill string is tripped to the surface. A LWD tool can provide a higher quality data than a wireline logging tool because it collects data during or shortly after drilling, before drilling mud invades the formation deeply. However, because of the large volume of data acquired by the tool, only a subset of the acquired data may be sent to the surface in real-time using a non-cabled connection, such as a measurements-while-drilling (MWD) telemetry system.
An image of the formation surrounding the borehole provides information about geological properties of the formation surrounding the borehole. As can be appreciated, if the borehole image is available in real-time, it can be interpreted and used to optimize downhole operations such as drilling. For example, parameters related to the direction and location of the drill bit can be derived from the image and used to geosteer the drill bit or maintain the drill bit within a desired pay zone. Data compression techniques, such as described in U.S. Pat. No. 6,405,136 issued to Li et al., may be used to reduce the size of the image data to facilitate its transmission to the surface in real-time via a MWD telemetry system. However, there are limitations on how much compression can be achieved without corrupting the data and/or the compression may not be significant enough to enable real-time transmission of all the image data needed to generate an accurate visual representation of the formation surrounding the borehole. An alternate approach to transmitting the image data to the surface in real-time is to calculate desired parameters downhole and then send the desired parameters to the surface in real-time. U.S. Pat. No. 5,899,958 issued to Dowel et al. describes a logging tool that acquires an image of formation surrounding a borehole for later retrieval. The logging tool includes a downhole processor that computes dip angle and dip direction from the image data. These parameters can be sent from the borehole to the surface in real-time.